If you are sitting across a table from someone, you don't see them as they are now, you see them as they were a few nanoseconds ago. Across a room it's more like ten or fifteen nanoseconds. One nanosecond is about a foot. The further something is away from you, the further back in time you are seeing. If you look at the Sun, you see it, not as it is now, but as it was eight minutes ago. If you look at he nearest star it's four years. But if you look out into space, past all the more recent objects, you see the Universe as it was when it was only three hundred million years old. This is when the contents of the Universe cooled from an opaque glowing plasma to transparent hot gas. It doesn't matter which direction you look, the whole Universe looked like that. It's the background to everything else.

But what about before that, when the Universe was opaque? The Universe didn't become transparent in one go at one instant. Small differences in density meant that some areas stayed opaque slightly longer than others. The result is tiny ripples in the brightness of that background. Just as you can look at the ripples on a pond and deduce what is in the pond, so you can look at the ripples in the background and deduce what is beyond it.

And those vast clouds of gas when gravity took over. I said that some of them collapsed and formed stars. Yes, but some of them didn't and they are still there. They're hard to see because they don't emit any light of their own, but we can see them from the light that passes through them from things further away. From the light they absorb we can see where they are and from the spectrum they absorb we can find out lots of things. From the relative spacing we can see they are mostly hydrogen, about a quarter helium with a smattering of lithium; from the position of the lines we know how far away they are, from the depth of the lines we can measure their density and from the width of the lines measure their temperatures.

What about those stars, those first primeval stars? To see that far back in time we have to look at things a very long way away and the brightest stars look very dim at that distance. We have seen one or two really enormous stars that are nearly old enough, so we may see some of those truly primordial stars any day now. More importantly, however, we have seen when those primordial stars turned on.

When the contents of the Universe were a glowing plasma all the atoms were ionised; that's what made it opaque. When it became transparent all the atoms were neutral, they all had the right number of electrons to balance the charge on the nucleus, and that's how it was through the period known as the Dark Age. But then when those primordial stars fired up they emitted large amounts of ultra-violet light and that light ionised some of the gas. This time. it didn't make an opaque plasma, it was far too thin, but nevertheless we can see when it happened. It was surprisingly early, when the Universe was about 500 million years old. (Everyone expected between 700 and 1,000 million, but surprises like this happen all the time when you allow reality to play a part in your thinking.)

But still, isn't it convenient that those star exploded, so we can't see them any more? Well sort of, but what about the other stars, the ones that didn't explode. The lifetime of a star depends on its mass. The bigger it is, the greater its gravity, the greater the pressure forcing it to collapse and therefore the greater the amount of heat that is needed to stop it collapsing. For most of its life a star balances on this gravity inward, heat outward equilibrium. It follows that really massive stars must use up their fuel very quickly to maintain the balance.

At the other end of the scale, very small stars, they don't have such great inward pressures, so they don't need such great outflows of heat, so they don't use up their fuel anywhere near as quickly. A really super-massive star, say 200 times the mass of our Sun, will live for two or three million years. Lesser stars, nowhere near as rare, will last for tens or hundreds of millions of years. Our own Sun has lasted 5,000 million years already and is about halfway through its life (and it's not going to explode at the of it, either). Smaller stars still can live for hundreds of billions of years.

Although the big stars of the past are gone and their elements mixed in with the interstellar medium, the little ones made at the same time as still around. So, when the Universe was young and few stars had exploded the stars produced then didn't have much in the ways of heavy elements. Later stars, when more stars had exploded, contained more. So what we see is that the oldest stars contain little of the heaver elements, younger stars contain more and the youngest stars, younger than our own Sun, contain much more than the Sun.

Oh yes, we have seen stars explode in just the way I have described. It's roughly once a century in one galaxy, but we have the remnants of such explosions in our galaxy and there's an awful lot of galaxies out there, so exploding stars are well studied.

That business about binding energies, about how you get most energy fusing hydrogen to helium, less fusing helium to carbon and so on down to fusing iron takes in energy rather than giving it out, you can get by measuring the atomic weights of the elements. These days it's easy using mass spectrometers and the like, but you can do it, and it originally was done, using the sort of equipment you can find in any high school chemistry lab. (Though you might get confused by oddities like chlorine.)

So, how do I know? Was I there? Yes. The ripples in the cosmic background tell me what happened when the Universe was very young. The transition from opaque to transparent I can see for myself, I can see those vast clouds of gas by the shadows they cast, I can see the build-up of heavy elements over time by looking at stars of different ages, I can watch stars explode and see how they mix heavy elements into the interstellar medium and I can support all this with measurements taken here on Earth.

So, yes, I was there.